1. Field of the Invention
The present invention relates to process and apparatus for electrophotography and more particularly relates to electrophotographic process and apparatus which improves the gradation (gradient) of electrostatic latent image and allows a color reproduction with improved color balance.
2. Description of the Prior Art
In the art of electrophotography, there have been made remarkable developments and improvements starting from the so-called Carlson Process in which an electrostatic latent image is formed on the surface of a photoconductive layer and then the formed latent image is developed for further use. As one of the important developments in this technical field, such electrophotographic process has been proposed and practically accepted in which a photosensitive medium having an insulating layer overlaid on a photoconductive layer is used and an electrostatic latent image is formed on the insulating layer. The latent image thus formed on the insulating layer is very stable against light.
Examples of such improved electrophotographic process are disclosed in U.S. Pat. No. 3,666,363 (GP 1,522,568), U.S. Pat. No. 3,734,609 and U.S. Pat. No. 4,071,361 all of which were proposed in behalf of the assignee of the present application. These processes employ the above mentioned multi-layer photosensitive medium and give particularly good electrostatic latent images of high stability and high contrast.
The photosensitive medium used in these electrophotographic process is formed by applying a photoconductive layer and a light transmissive dielectric layer on a support of electrically conductive or insulating material. In a dark place or in a light place, the photosensitive medium is charged with a corona discharge so that electric charges may be trapped in the interface between the photoconductive layer and the light transmissive insulating layer or in its neighbouring area. Then, a light image is projected onto the surface of the photosensitive medium while applying to it a corona discharge of the opposite polarity to that of the firstly applied corona discharge or an alternating corona discharge. This secondary corona discharge is followed by a whole surface exposure of the photosensitive medium so that making use of difference in impedance of the photosensitive medium between the dark and the light, the electric charge on the light part may be reversed or cancelled out. In this manner, a latent image is formed which has a contrast in electrostatic potential. The latent image thus formed is developed and transferred in the conventional manner to make a photoprint. In case of color reproduction, individual elemental color images of the color original are produced by known color separation technique and they are overlaid on each other to form an overlaid color print. More particularly, this color electrophotographic process involves the following steps:
Initially, the photosensitive medium is charged by corona discharge. Then, a light image of a color original is projected onto the photosensitive medium through a color filter in one of three primary colors of additive color process, for example, through a blue color filter while effecting corona discharging with the opposite polarity or with alternating current. Thereafter, the photosensitive medium is subjected to a whole surface exposure so as to form an electrostatic latent image thereon. The latent image thus formed is developed with color toner whose color is one of the three primary colors of subtractive color process and also complementary color to the color of color filter used, namely, in this case with yellow toner. The developed image is transferred onto a suitable support such as a sheet of paper. In this manner, at first there is formed an elemental color image of blue component of the color original.
In the same manner, a green component image is formed using a green filter for exposure and magenta toner for development. The green component image developed in magenta is properly registered with the firstly formed blue component image developed in yellow and the former is overlaid on the latter by overlapping transference.
Lastly, a red component image is formed in the same manner using a red filter for exposure and a cyanic toner for development. The red component image developed in cyan is registered with and transferred onto the above component image developed in magenta so that an overlaid color print is finally produced.
If necessary, so-called black print may be overlaid further onto the color print to improve the quality of the formed color image. For this purpose, a latent image is formed using a ND filter and then developed with black toner. The developed image is registered with the color print and transferred by overlapping transference.
In this manner, monochromatic image or multicolored image corresponding to the original is obtained using this electrophotographic process. The electrostatic latent image formed in this process is featured by its higher contrast as compared with other known processes. In general, the higher the contrast is, the more easy reproduction of gradation is allowed in this process. However, it is still a difficult problem to reproduce the gradation by a satisfactory degree.
This problem of gradient will be understood from the characteristic curve of latent image potential (V)--exposure (E) shown in FIG. 1.
FIG. 1 shows the characteristic curves of individual color component images in three-color separation obtained in the above described color reproduction. The degree of gradation of image is represented as the gradient of the curve and it is generally called ".tau. value". This .tau. value is very difficult to control. For example, in the above described process there was never obtained any satisfactory result even when potential of corona discharge and amount of exposure were variously controlled. Particularly in color reproduction, this difficulty of control or .tau. value brings forth a particular important problem against good image reproduction. As well-known in the art, in color reproduction there is allowed to obtain an image of good color balance only when the conformity in gradient (.tau. value) of all the three elemental color images is attained. But, in practice, as seen best in FIG. 1, three individual color component images formed on the photosensitive medium are generally different from each other in .tau. value.
As will be understood from FIG. 1, the characteristic curve (B) obtained at the time of blue exposure has such tendency that the potential at the light portion and its neighbouring portion becomes higher. Characteristic curve (G) of green exposure exhibits somewhat similar tendency. For this reason, these two curves are out of balance relative to the characteristic curve (R) of exposure in red color. As a result, final .tau. values of three color component images in image density--original density curves become different from each other and therefore no good balance in color can be obtained. The component image exposed to blue light and developed in yellow is apt to suffer from fogging. All the attempts to true up the three characteristic curves of latent image potential (V)--exposure (log E) in three-color separation exposure have resulted in failure. Even when the potentials of the primary charging, of the secondary charging with the opposite polarity or of AC discharging or the value of exposure in color separation were variously changed, there could be obtained no satisfactory result.